1. Field of the Invention
The present invention relates to a code modulation method, a code demodulation method and a code detection method for being suitably used in digitally recording, for example, audio data or video data to and playing back the data from a recording medium such as magnetic tape etc.
2. Description of Related Art
When video or audio data is digitally recorded on or played back from a disc such as a magnetic disc, an optical disc, or a magneto-optical disc etc. or magnetic tape, it is preferable that the data is, if possible, recorded at a high density. A partial response (1, 0, -1) or a partial response (1, 1) etc. is usually used for this purpose. High density recording is then possible by adopting a PRML (Partial Response with Maximum Likelihood) method that combines the partial response (1, 0, -1) or the partial response (1, 1) and Viterbi detection.
For example, with the partial response (1, 0, -1) (hereinafter abbreviated to PR4), when the free squared-Euclideam distance (hereinafter referred to as the free distance) during the conventional bit-by-bit detection method is taken to be 1, the free distance at the time of detection can be taken to be 2 via Viterbi detection. Signal to noise ratio (hereinafter referred to as S/N) can then be improved by having the value for the free distance large and the recording density can therefore be made correspondingly high. Further, with PR4, a free distance of 4 is possible during Viterbi detection, by the code modulation method.
However, it is necessary during playback that the modulation code does not include a D.C. component to make sure that, for example, errors due to variations in a reference level during binarization (digitization) of the playback signal do not occur or, for example, to ensure that fluctuations do not occur in each type of error signal such as tracking error signals etc. occurring in the so-called "servo control" of disc devices.
With the data symbols [1] and [0] being taken to be +1 and -1, the so-called DSV (Digital Sum Value), that is the sum total of the symbols from the time of starting the modulation code list, is a standard for evaluating the aforementioned D.C. component, with a small absolute value for DSV meaning that there is little D.C. component or low frequency component.
In modulation such as the 8-10 code adopted in digital audio tape recorders (DAT), the EFM (Eighteen to Fourteen Modulation) adopted in compact disc (CD) players and the Miller 2 (Miller squared) modulation adopted in magnetic disc devices, DSV control is exerted to make the absolute value for DSV small after, for example, NRZ (Non Return to Zero) present in so-called mark position modulation or NRZI (Non Return to Zero Inverted) present in mark length modulation is carried out.
With PR4, the characteristic of PR4 does not have a D.C. component and DSV control to make the absolute value of the DSV of the modulation code string small is therefore not needed. However, with the partial response (1, 1) (hereinafter abbreviated to PR1), the characteristic of PR1 does have a D.C. component and DSV control does have to be carried out for the modulation code list.
Further, the path memory length during Viterbi detection is a problem when adopting the PRML method. The path memory has a length proportional to the time interval until results are fixed at a storage device that pre-stores assumed detection values until the Viterbi detection results are decided. The time interval until the Viterbi detection results are decided i.e. the path memory length is controlled in accordance with the way of making the modulation code. For example, when applying 8-10 code to PR1, the maximum value of number of one continued in the modulation code string before NRZI modulation decides the necessary path memory length.
In reality, however, a device having an unlimited path memory is not possible, and a small path memory is necessary from the point of view of cost and the amount of occupied space. Control therefore has to be carried out with the aim of making the path memory length short for the modulation code string.
However, with the PR1, which has a small amount of spectrum in high-frequency region and is suited for having a high density when compared with the PR4, by using the improved 8-10 modulation code disclosed in the previous application of SN=08/309981 (Japanese Laid-Open Patent No. Hei 7-93906) by the same applicant, free distance during detection can be made to be 2 and the path memory can be made shorter while carrying out DSV control. But, the free distance at the time of detection while carrying out DSV control cannot be made larger than 2 even with this improved 8-10 conversion and the path memory length cannot be made further shorter.